Title: Sediment Study "Good News" for Quake-Prone Southern California
Date: December 10, 1997
Embargoed until 11 A.M., PST
Media contact: December 10, 1997
Cheryl Dybas NSF PR 97-74
(703) 306-1070/cdybas@nsf.gov
Program contact:
Jim Whitcomb
(703) 306-1556/jwhitcom@nsf.gov
SEDIMENT STUDY 'GOOD NEWS' FOR
QUAKE-PRONE SOUTHERN CALIFORNIA
The Los Angeles basin's sediments seem to lessen the ground
motion that threatens single-story and low-rise buildings in a
severe earthquake, a new study of data from the 1994 Northridge
quake has revealed. The study was conducted through the National
Science Foundation (NSF)'s Southern California Earthquake Center.
University of Southern California seismologist Edward H.
Field says that his team's research, which is published in this
week's issue of the journal Nature, is "good news" for Southern
California.
"This work is an important step in our efforts to mitigate
the damage caused by earthquakes," says James Whitcomb, program
director in NSF's division of earth sciences, which funds SCEC.
"Now, we must categorize the geological conditions and building
types for which this phenomenon holds true. That's the cutting
edge of future earthquake research."
Geologically, the Los Angeles basin is a valley filled with
debris (sediments) that eroded from neighboring mountains over
hundreds of thousands of years, Field notes. For more than a
century, scientists have known that such sediments usually
amplify ground motion in earthquakes. But seismologists and
engineers disagree as to whether the degree of amplification will
change as the level of shaking increases.
Do all sediment-filled valleys shake like a bowl of jelly in
larger earthquakes, as they do during smaller quakes? Or do some
behave like a bowl of sand in which seismic energy is "absorbed"
as the grains rub together, effectively reducing ground motion?
Based on laboratory studies of sediments, engineers have
argued for the bowl-of-sand theory and have designed structures
on the assumption that amplification factors go down as the level
of shaking increases - that is, that the shaking effects of a
stronger earthquake aren't boosted as much as those of a smaller
quake. Seismologists, though, have traditionally argued for the
bowl-of-jelly model. They have seen little evidence that
sediment amplification is reduced, especially when the soil is of
the stiff, dry variety found in the Los Angeles basin. They have
therefore been concerned that some engineering designs may fail
to account for the degree of seismic hazard that sediments
actually pose.
The new study provides the first evidence based on large-
scale measurements that the answer is closer to the engineering
view than seismologists had thought. Sediment amplification in
the Los Angeles basin would be significantly reduced during large
earthquakes, the researchers found. Although the Nature paper
takes a step toward settling the debate, the
researchers believe more work is needed to assess whether current
engineering practices adequately reflect the degree of seismic
hazard posed by the local sediments.
-NSF-
Editors: Field will attend the meeting of the American
Geophysical Union in San Francisco during the week of
publication. He may be reached at the Westin St. Francis Hotel
at (415) 397-7000.
***NSF is an independent federal agency responsible for
fundamental research in all fields of science and engineering,
with an annual budget of about $3.3 billion. NSF funds reach all
50 states, through grants to more than 2,000 universities and
institutions nationwide. NSF receives more than 50,000 requests
for funding annually, including at least 30,000 new proposals.